A typical refrigeration control system consists of a compressor to compress gaseous refrigerant, a condenser in which the refrigerant liquefies, an evaporator in which the liquid refrigerant transforms back to a gaseous state, an accumulator which collects refrigerant, and an adjustably controllable expansion valve to control the flow of liquid refrigerant to the evaporator. The expansion valve is controlled to maintain a constant superheat value such that high thermal efficiency is achieved while ensuring that liquid refrigerant does not flow out of the evaporator which would be potentially damaging to the compressor. Superheat is defined as the difference between the vapor temperature of the refrigerant and the saturation temperature of the refrigerant when it is at its liquid-vapor transition point. Two sensors are required to determine the superheat value at the exit of the evaporator. One sensor is used to obtain the vapor temperature and the other sensor is used to obtain the saturation refrigerant temperature.
The expansion valve is controlled by electronic means which senses the superheat value and correspondingly provides an output to change the opening or closing of the valve and hence maintain the value of sensed superheat to a set point. In order for the electronic control means to be highly responsive to adjust for start-up transients and for variations in parameters during steady state operation, an unfiltered, high control gain is provided by the control.
Problems occur in this prior art control mechanism in that the high control gain may result in hunting of the superheat value because of the time lag between changes in the expansion valve and corresponding changes in superheat. Hunting is defined as the condition when the superheat value, instead of stabilizing, oscillates continuously about the set point, resulting in decreased efficiency and possible passing of liquid refrigerant to the compressor.